1. Field of the Invention
The present invention relates to aqueous resin based inkjet inks, more specifically aqueous inkjet inks containing capsules, such as microcapsules or nanocapsules.
2. Description of the Related Art
Over the last years, offset and flexographic printing systems are being increasingly replaced by industrial inkjet printing systems due to their flexibility in use, e.g. variable data printing, and due to their enhanced reliability allowing their incorporation into production lines.
Radiation curable inkjet inks have been the preferred choice of ink for reasons of reliability and because high quality images can be printed on non-absorbing ink-receivers. However for economical and ecological reasons, it is desirable to be able to print aqueous resin based inks in a reliable way on these industrial inkjet printing systems.
It has also been observed that the required physical properties of the printed image such as adhesion performance, scratch resistance, solvent resistance, water fastness and flexibility, are much more difficult to obtain by aqueous inks compared to reactive inks.
Encapsulation is a process in which tiny particles or droplets are surrounded by a shell to give small capsules. The material inside the capsule is referred to as the core or the internal phase, whereas the shell is sometimes called a wall. This technology has been applied in different technical fields, such as self healing compositions (Blaiszik et al., Annual Review of Materials, 40, 179-211 (2010)), textile treatment (Marinkovic et al., CI&CEQ 12(1), 58-62 (2006); Nelson G., International Journal of Pharmaceutics, 242, 55-62 (2002), Teixeira et al., AIChE Journal, 58(6), 1939-1950 (2012)), thermal energy storage and release for buildings (Tyagi et al., Renewable and Sustainable Energy Reviews, 15, 1373-1391 (2011)), printing and recording technology (Microspheres, Microcapsules and Liposomes: Volume 1: Preparation and Chemical Applications, editor R. Arshady, 391-417 and ibid., 420-438, Citus Books, London, 1999), personal care, pharmaceuticals, nutrition, agrochemicals (Lidert Z., Delivery System Handbook for Personal Care and Cosmetic Products, 181-190, Meyer R. Rosen (ed.), William Andrew, Inc. 2005; Schrooyen et al., Proceedings of the Nutrition Society, 60, 475-479 (2001)) and electronic applications (Yoshizawa H., KONA, 22, 23-31 (2004)).
The use of encapsulation technology in ink jet inks has largely been limited to the design of encapsulated pigments, where a polymer shell is directly polymerized on the surface of the pigment particles. For example, US 2009227711 A (XEROX) discloses encapsulated nanoscale particles of organic pigments, comprising a polymer-based encapsulating material, and one or more nanoscale organic pigment particles encapsulated by the polymer-based encapsulating material to be used as colorants for compositions such as inks, toners and the like. This approach doesn't allow boosting the physical properties needed in industrial applications.
JP 2004075759 (FUJI) discloses an ink jet ink including a microcapsule comprising at least one hydrophobic dye, at least one hydrophobic polymer and at least one high boiling solvent, where the capsule walls are prepared using a polyfunctional isocyanate compound. All the examples disclosed require the use of an additional water soluble polymer, i.e. gelatine.
Encapsulation as an approach to integrate reactive chemistry in ink jet inks has hardly been disclosed. US 2012120146 A (XEROX) discloses a curable ink comprising microcapsules. The microcapsules contain at least one first reactive component and at least one second component comprising a triggerable compound, and they are dispersed in at least one third reactive component. After stimulus induced rupture of the capsules, polymerisation of the ink is obtained by reaction of the at least one first reactive component with the third reactive component. From Example 6, it should be clear that the microcapsules are integrated into a UV curable ink rather then an aqueous based ink.
US 2014002566 A (SEIKO EPSON) discloses an inkjet ink including a coating film forming material, a polyether-modified silicone oil, and water, resulting in micelles dispersed in an aqueous medium. In one preferred embodiment the inkjet ink is a photocurable inkjet ink by including a photocurable compound in the micelles. A similar concept is disclosed by US2011237700 A (SEIKO EPSON).
US2011261108 A (TOSHIBA TEC) discloses a decolorizable water-based inkjet ink including a color material, a solvent, and a nonionic surfactant, wherein the color material contains a color developable compound and a color developing agent.
Reviewing the synthetic approaches for the synthesis of microcapsules in general, it becomes clear that the use of an additional hydrophilic polymer is required to control the colloid stability, the particle size and the particle size distribution, which are three critical factors for the design of an ink jet ink. However, the use of water soluble polymers in aqueous based ink jet inks very often has a detrimental impact on jetting reliability and latency, aspects which are particularly important in an industrial environment where down time and complex maintenance cycles have to be avoided.
Therefore, there remains a need for aqueous resin based inkjet inks exhibiting good physical properties on a wide range of substrates, while exhibiting high reliability for industrial inkjet printing.